The present invention relates to a fuel cell that is formed of an anode, electrolyte membrane, cathode, and diffusion layer and is configured so that fuel is oxidized on the anode and oxygen is reduced on the cathode. Also, it relates to a generator incorporating such a fuel cell, a small-size portable power source, and electric or electronic equipment using such a power source.
With advances in electronic technology, electric and electronic equipment such as telephone sets, book type personal computers, audio and visual equipment, and mobile information terminals and small-sized portable electronic equipment have come into wide use rapidly.
Conventionally, such portable electronic equipment has been a system driven by a primary battery or a secondary battery. The secondary battery has developed with emergence of new type secondary batteries, decreased size and weight of battery, and the high energy density technology. The new type secondary batteries have developed from sealed lead batteries to nickel-cadmium batteries, nickel-hydrogen batteries and further to lithium-ion batteries. However, the secondary battery must be charged after a fixed amount of electric power has been consumed, so that a charging facility and charging time are needed. Therefore, there remain many problems in driving the portable electronic equipment continuously for a long period of time. In the future, the portable electronic equipment will tend to necessitate a power source with a high energy density, that is, a power source capable of withstanding long-term continued use in response to increased information amount and increased speed. Therefore, a need for a small generator (micro-generator) that need not be charged is increasing.
To fulfill this need, a fuel cell power source has been proposed. The fuel cell converts chemical energy of fuel directly into electrical energy in an electrochemical manner. It does not necessitate a power section using an internal combustion engine such as an ordinary engine generator, and has a possibility of being used as a small power generating device. Also, the fuel cell can continue power generation merely by replenishing fuel, so that it can eliminate the need for stopping driving of portable electronic equipment in use for the purpose of charging unlike the conventional secondary battery.
Among these fuel cells, a polymer electrolyte fuel cell (PEFC), in which by using an electrolyte membrane of perfluorocarbon sulfonic acid, hydrogen gas is oxidized on the anode and oxygen is reduced on the cathode to generate power, is known as a cell with a high output density.
To make the fuel cell of this type smaller in size, as disclosed in, for example, JP-A-9-223507 specification, a small PEFC generator has been proposed in which an assembly of cylindrical cells provided with anode and cathode electrodes on the inside and outside surfaces of a hollow fiber shaped electrolyte is formed, and hydrogen gas and air are supplied to the inside and outside of the cylinder, respectively.
In the case were this fuel cell is used as a power source for portable electronic equipment, however, the volumetric energy density is low because the fuel is hydrogen gas, so that the capacity of a fuel tank must be increased.
Also, this system requires auxiliary equipment such as equipment for feeding fuel gas and oxidizer gas (air etc.) into the generator and equipment for humidifying the electrolyte membrane to maintain the cell performance, so that the generating system has a complex construction, and cannot be made small in size sufficiently.
In order to increase the volumetric energy density of fuel, it is effective to use liquid fuel and to make the construction simple by eliminating the auxiliary equipment for supplying fuel and oxidizer to the cell. For this purpose, some proposals have been made. As a recent example, a direct methanol fuel cell (DMFC), in which methanol and water are used as fuel, as disclosed in JP-A-2000-268835 specification and JP-A-2000-268836 specification has been proposed.
This generator is configured so that a material for supplying liquid fuel by the capillary force is provided on the outside wall side of a liquid fuel tank, an anode is disposed so as to be in contact with the material, and further a polymer electrolyte membrane and a cathode are bonded successively. Oxygen is supplied by the diffusion of oxygen to the cathode outside surface touching the outside air. According to this system, therefore, the generator has a simple configuration that does not require the auxiliary equipment for supplying fuel and oxidizer gas.
However, since the output voltage of DMFC at the load time is 0.3 to 0.4 V per unit cell, it is necessary to mount fuel tanks attached to the fuel cells, the number of the fuel cells corresponding to the voltage required by portable electronic equipment etc., and to connect the cells to each other in series. Therefore, there arises a problem in that to make the size of generator small, the capacity of fuel tank decreases with increasing number of cells connected in series, so that the number of fuel tanks is distributed according to the number of cells connected in series.